With the advancement of LEDs into broad sectors of general lighting, there is the need for particularly simple and inexpensive power supply circuits for these component parts. A basic circuit for LEDs which is in widespread use is a mains-operated buck converter, as is illustrated by way of example in FIG. 1. The buck converter 10 illustrated has an input with a first input connection E1 and a second input connection E2, which can be coupled directly to an AC supply voltage UN. The inputs GE1, GE2 of a rectifier 12 are coupled to the input connections E1, E2, said rectifier comprising the diodes D1 to D4. In this case, the buck converter 10 is operated on an AC supply voltage, and therefore a rectifier is provided. Alternatively, the buck converter 10 can also be operated directly on a DC supply voltage, as a result of which the rectifier is then no longer needed.
A capacitor C1 with small dimensions in respect of a high mains power factor in comparison with the load operating on the buck converter is coupled between a first output connection GA1 of the rectifier and a second output connection GA2. A voltage divider which comprises nonreactive resistors R1 and R2 is connected in parallel with the capacitor C1. The tap of the voltage divider R1, R2 is supplied to a control apparatus 14, wherein the parallel circuit comprising the nonreactive resistor and a capacitor C2 is used for filtering a voltage present at the input E1, E2, with the result that reliable information on the present value of the input voltage UN is available at the input FE. In order to supply the control apparatus 14, said control apparatus is likewise coupled to the rectifier outputs GA1, GA2. The buck converter 10 comprises a buck diode D7 and a buck switch S1, which are coupled in series to form a current measuring resistor R3 between the rectifier outputs GA1, GA2. The voltage drop UR3 across the resistor R3 is supplied to the control apparatus 14 via a resistor R4. The control apparatus 14 is designed to actuate the control electrode of the switch S1 via a resistor R5. The actuation takes place in particular in such a way that the buck converter 10 is operated in a discontinuous mode. The operating frequency is preferably between 20 and 500 kHz, for example 65 kHz.
The buck converter 10 has an output with a first output connection A1 and a second output connection A2, between which a multiplicity of series-connected LEDs D8, D9, D10 are coupled in this case as load. The number of LEDs coupled to the output A1, A2 is preferably between 1 and 20.
A buck inductor L1 is coupled between the coupling point N1 of the buck diode D7 and the buck switch S1, on one side, and the output connection A2, on the other side. In the charging phase of the buck converter, a current flows in the circuit GA1, A1, D8, D9, D10, A2, L1, N1, S1, R3, GA2. In this case, the buck inductor is magnetized. In the discharging phase, the buck switch S1 is closed and the buck inductor L1 is de-magnetized. Therefore, a current flows in the circuit L1, D7, A1, D8, D9, D10, A2.
In the buck converter 10 illustrated in FIG. 1, the power supply to the control apparatus 14 takes place via the input HV. This power supply is dissipative. In view of the fact that such control apparatuses have a current consumption of a few mA, this procedure is undesirable for steady-state operation. Instead, a simple power supply to the control apparatus 14 with as few losses as possible is desirable.
In this context, it is known to implement an auxiliary power supply for the control apparatus 14 by virtue of the fact that an auxiliary winding is provided on the buck inductor L1. However, this results in the disadvantage that the buck inductor L1 becomes a special component part, thereby undesirably increasing the implementation costs for specific applications.